Prevent Okular from crashing when synctex logs an error

bd20e48c updated Okular's copy of the synctex code to 1.19.
Unfortunately since this version the upstream code logs errors like
`"! SyncTeX Error : Ignored record...` when accessing selected synctex
files created with older versions of synctex.

The upstream `_synctex_log` contains `va_list arg` as a parameter, but
fails to initialize and tear down this properly via `va_start` and
`va_end`. In general this seems to work for the single argument case.
However, once we apply our hardening patch to get rid of the
`gnu_printf format attribute` warning and thus introduce a variadic
argument, things go wrong.

To fix this, we add the missing code. The remaining changes are just
refreshing the patches.

BUG: 383915

Test Plan:
Opening `empty.pdf` with `empty.synctex.gz` from
https://bugs.kde.org/attachment.cgi?id=109116 located
in the same folder does not lead to Okular segfaulting anymore.